How much does baryonic feedback suppress small-scale matter clustering?
In plain words
Black holes and exploding stars blow gas out of galaxies, which smooths the matter distribution on small scales. If this effect is stronger than assumed, it could explain the low clumping seen by some surveys.
Precise statement
Determine the ratio $P(k) / P_{\mathrm{DMonly}}(k)$ of the total matter power spectrum to the dark-matter-only one for $k = 0.1 \text{ to } 10\,h/\mathrm{Mpc}$ at $z < 1$, with 1 to 2 percent precision. Use kinetic and thermal Sunyaev-Zeldovich profiles, X-ray gas fractions and lensing; state whether the suppression at $k \sim 1\,h/\mathrm{Mpc}$ is large enough (of order 20 to 30 percent, moderate confidence on this figure) to bring lensing S8 into agreement with the CMB.
What would settle it
Stacked kinetic Sunyaev-Zeldovich gas profiles around DESI galaxies and clusters, jointly fitted with cosmic shear.
Status in the literature
Unverified note
2024 to 2025 stacked kinetic Sunyaev-Zeldovich measurements indicate gas more extended than in several standard hydrodynamic simulations (moderate confidence).